At best an online test with good cans is going to show the relative frequency loss, without saying much about the actual amount of the deficit. That could still be useful for screening, and alert someone who hasn't a clue like the lady in the following anecdote:
An elderly woman joined the doctor following his examination.
"Are there any concerns you would like to discuss?" he asked. as was his routine.
"Well yes, doctor," she said, "I seem to have a problem with farting. You know the type silent but deadly."
"Yes madam," he replied, "I do know. And it appears you also have a problem with your hearing as well."
It is not too difficult to find a free audiogram using calibrated equipment in a sound attenuated booth in most cities in the US. Typically they are offered by people who are trying to flog hearing aids, but in many cases they are nonethelss accurate. Even hearing aid fitters have the occasional bout of ethics.